In order to detect and locate cracks and other surface defects in slabs for rolling steel flats, suitable sensors are used. These sensors (based on eddy currents, for example, according to DE-OS No. 20 44 331) must usually be disposed closely (e.g., 4 mm) above the surface and they have only a narrow effective range. To be able to scan larger surface areas with a sensor, the sensor can be moved back and forth within a certain range over the slab emerging continuously from a strand casting apparatus. This movement can be performed, for example, by a crank drive (DE-OS 27 46 618) which makes a carriage running on guides shift back and forth.
For the scanning pattern to be narrow enough, the reciprocating movement of the sensors must be fast, and the mass forces which this produces increase as the square of the scanning frequency and can become very great.
It is, therefore, an object of this invention to provide a surface testing apparatus by means of which the sensors can be moved back and forth over the surface with a high scanning frequency.
A further object is to minimize the forces which are required for the movement of the sensors.
A further object is to provide a surface testing apparatus with a vibratory system for moving the sensors and for operating the vibratory system at or close to its natural frequency.
According to the invention it is proposed, for moving the sensors back and forth, to employ an oscillating system which is driven by means of an exciter system at or close to its natural frequency. Since the distance of the sensors from the surface being examined must be kept within close tolerances, it is additionally proposed to control the height and lateral inclination of the sensors by the deformation of resilient elements or to hold the sensors at a constant distance from the surface under examination by means of biased pneumatic or hydrostatic thrust bearings.
Additional advantageous features of the invention will be found in the subordinate claims.
Embodiments of the invention will be seen in the drawings.